1. Field of the Invention
The field of art to which this invention pertains is biaxially oriented thermoplastic films. The invention is further directed to a method of making such films.
More specifically, in a preferred embodiment, a relatively thin polyester film is biaxially stretched simultaneously in a tenter frame to provide an exceptionally thin film having improved gauge (thickness uniformity) and improved freedom from process interruptions due to film breaks during stretching. The film is maintained within an orientation temperature range, by use of radiant heaters, during the stretching step. Ultrathin films having a thickness of less than 2.5 microns can be readily manufactured using this method.
2. Description of the Related Art
It is well known in the art that some physical properties of thermoplastic films, including tensile strength and modulus of elasticity, may be improved by stretching the film.
The stretching techniques which have been previously used vary widely depending on the type of films involved, the methods of making those films, the properties being sought and other factors. For example, in a typical process, the film, in the form of a continuous web, may be stretched either sequentially using rollers for machine direction stretching and a tenter frame for cross-machine stretching, or simultaneously in an appropriately equipped tenter to properly orient the film and thereby improve its properties. In other instances, the stretching operation may be carried out by casting film in tubular form and stretching in an appropriate apparatus using a combination of tubular expansion and machine-direction stretching techniques.
The present invention is specifically directed to an improved method of making ultrathin biaxially oriented films in a tenter frame. Such films have acceptably uniform gauge with a minimum of gauge variations in the transverse and longitudinal directions. The films are ultrathin, having a thickness of 2.5 microns or less.
In the stretching method of this invention, a molten polymer is extruded onto a quenching surface through an orifice and cooled to form a continuous web of film which is then stretched to form the desired film product. It is very difficult, in the manufacture of polymeric film formed by extrusion and stretching of a film web, to achieve sufficiently uniform thickness over the entire web. The as-cast film always contains numerous narrow zones or regions extending in the longitudinal or machine-direction of the film web which are thicker or thinner than the mean of the film. These zones or gauge bands result from a variety of causes including differences in temperature or viscosity of the extruded polymer melt, variations in roughness or wettability of the extrusion die lip and for other reasons. This variation in thickness, referred to as transverse gauge variation, is measured by scanning the thickness profile of the film in the transverse or cross-machine direction. Variations in thickness also are known to occur in the film web in the machine direction due to periodic vibration of equipment, variability in polymer pumping rate, or nonuniform stretching conditions, among other causes.
It further is known that stretching thermoplastic film by conventional methods generally causes magnification of gauge variations, or an increase in the percentage deviation from the mean or average thickness of a film in certain areas or zones of the film. This gauge variation magnification (i.e., increase in the ratio of the thickness of thick areas to the mean thickness and conversely, a decrease in the ratio of the thickness of thin bands or areas to the mean thickness), occurs upon applying stretching tension to the film because at a given temperature the thinner areas may be stretched more easily than the thicker areas so that differences in thickness are greater after stretching than before. The thin areas tend to stretch and decrease in thickness while the gauge of thicker portions may decrease substantially less, thus, gauge differences or variations between the thick and thin portions of the film are magnified.
In conventional practice it is known to heat a thermoplastic film web to a temperature at which it may be more readily oriented, prior to stretching. It is further known that the thinner regions of the web, as cast, tend to be raised more rapidly to a higher temperature than the thicker regions while being heated to an appropriate stretching temperature. This heating step tends to even further increase or magnify the gauge variations in the film since the thinner regions or portions will stretch even more during the stretching operation.
Therefore it is advantageous to follow the casting of orientable polymer films with a stretching technique which not only does not magnify, but preferably reduces, gauge variations in the stretched film. The present invention provides such a critical combination of conditions of stretching in which gauge variation magnification is substantially reduced or minimized.
More specifically in producing the ultrathin thermoplastic films of this invention (i.e., films having a final thickness of 2.5 microns or less) it has been found that improved gauge may be obtained in the stretched film by using a method in which the thicker regions or portions of the film are at a higher temperature than the thinner portions during stretching. It further is critical in the practice of this method that the appropriate temperature differentials between thick and thin portions be maintained during the stretching operation.
In the film making techniques of the prior art, hot-air heaters are typically used to heat the film. The film temperature is maintained or adjusted by impingement of hot air on the film. In the hot-air method of heating, heat input is proportional to the surface area and independent of film thickness. Thus, the thinner areas of the film tend to be heated faster because of their reduced mass per unit area compared to the average thickness, and temperatures of thicker areas tend to rise more slowly because of their greater mass per unit area. If stretching occurs before the entire film reaches temperature equilibrium, the thinner areas (which are at a higher temperature relative to the thicker areas) tend to stretch more, resulting in magnification of the gauge variation problem and, in the case of thin film, film breakage due to overstressing of the thinner regions. If the film is stretched after the film temperature has fully equilibrated with the air temperature then gauge magnification may still occur.
It is also known to use infrared or radiant heaters are used to heat the film either prior to or during stretching. Hot-air heaters are sometimes used in combination with radiant heaters. Further, various cooling and film pretreating techniques have been employed either during or prior to heating and stretching, all to improve the properties of the film. Typical examples of these techniques are described in U.S. Pat. No. 3,231,642 to Goldman et al and U.S. Pat. No. 3,510,552 to Tsuruta et al, for example. None of these stretching methods are suitable for stretching thin films in the thickness range of the present invention. They offer no means to maintain the temperature differentials of the thicker and thinner areas of the film within their proper stretching ranges or require additional steps, such as cooling during stretching, which would severely impact the operation of the method of the present invention and tend to render it inoperative.
In the Goldman patent, a relatively thick tubular film is extruded through an orifice, exposed to radiant heat and then stretched while being cooled. The cooling air in the stretching zone serves to cool the thin portions of the film faster, reinforcing them, and thus improving gauge uniformity. The tubular film, as cast, has a wall thickness of anywhere from 15 to 85 mils (e.g., from 381 to 2159 microns thick) and is stretched, while cooling, to a thickness ranging from 0.8 to 1.2 mils thick (e.g., 20.3 to 30.5 microns thick).
The Goldman process of heating the film to higher temperatures then cooling during stretching (utilizing the faster cooling rate of the thinner regions to create transient temperature differentials between thinner and thicker areas) would not be operative with thin films because these films would too quickly reach uniform temperature.
Further, in the process taught in the Tsuruta et al patent, 2 to 12% by weight of water is added in a pretreatment step to a relatively thick polyamide film prior to heating by conventional hot-air and infrared heaters. The film is then drawn at a temperature at least 30.degree. C. below the melting point of the film to improve its gauge. The water is used to allow significantly greater elongation in the stretching process than is possible without it. A plasticizer is not required in the manufacture of ultrathin films of the type made by the method of the present invention.
The films stretched by the method of the present invention are sufficiently thin (i.e., under 25 microns) prior to stretching so that the absorption of energy, from the radiant heaters, is nearly proportional to thickness. Convective heat loss to the surrounding atmosphere is proportional to the film surface area. As a result, the thicker regions become hotter than the thinner regions such that the temperature differential at any point in the film is in an amount proportional to thickness at that point.
It is critical in the practice of this invention that the temperature differentials between regions of varying thickness be maintained at their proper levels during stretching. It has been found that these temperature differentials may be effectively maintained at appropriate levels, for these thin films in thermal equilibrium with surrounding atmosphere by the continued application of radiant heat. That discovery is the essence of this invention.
This condition would not apply to thicker films, such as those taught in Goldman et al patent. For such films, the energy absorbed by radiant heating is significantly less than proportional to thickness and temperature equilibrium is reached more slowly because of the greater mass per unit area. In that process the entire film must be heated above a desired temperature, then cooled during stretching so that the thinner regions become cooler than thicker ones for a brief time during which stretching occurs.
The Goldman process of heating and cooling the film would not be suitable for stretching ultrathin films since the desired temperature differentials could not be maintained between thicker and thinner portions of the film because rapid equilibration with the forced cooling air during the stretching step would cause the film temperature to become uniform. It is critical in the practice of the present invention that the thicker portions of the film remain at a higher temperature than thinner areas during the stretching step.
The present invention provides means to establish a temperature differential of at least 0.1.degree. C. higher in regions of the film which are 5% thicker compared to the average thickness (and, conversely, temperature differentials of at least 0.1.degree. C. lower in regions 5% thinner than average) before stretching begins or early in the stretching process. It is preferred that the appropriate temperature differentials be established prior to stretching the film beyond its elastic limit, about 3% elongation for polyethylene terephthalate. The temperature differentials between the thicker and thinner regions of the film are established by a localized heat balance where the heat input is controlled by the film thickness and the absorption characteristics of polymer for the energy flux from the radiant heaters while the heat loss to the surrounding atmosphere is controlled by air movement. It is preferred to minimize the air movement in the vicinity of the film web in order to maintain the proper temperature differentials without moderating the temperature difference between the thick and thin regions. Alternatively, air flow in a direction parallel to and, preferably concurrent with and at a similar speed as, the film is also acceptable. It is critical to the practice of this invention that the desired temperature differentials are not eliminated by excessive air movement during stretching. Forced air impingement against the thin film web will diminish or eliminate the temperature differential between the thicker and thinner regions of the film.
For these and other reasons, it has long been known to the art that it is extremely difficult to manufacture ultrathin films, particularly those having a final thickness of 2.5 microns or less. It is not only difficult to obtain acceptably uniform gauge in the final film product, but film breaks commonly occur during processing. Gauge variations in the extruded film web are a prime cause of this problem. As previously indicated, the thinner portions of the film are generally weaker than the thicker portions and during stretching they become progressively thinner and tend to break creating runnability problems with the stretching operation. This is particularly noticed when producing thin film in the form of a continuous web which is to be wound into a roll. Breakdowns occur so frequently that it is very difficult to produce a wound roll web having a length of at least 5000 meters.
Prior art processes to improve runnability in ultrathin film manufacture have focussed primarily on methods wherein a laminate film structure is produced by coextrusion or lamination, the laminate is oriented by stretching, and subsequently delaminated to provide the ultrathin film. Difficulties are frequently encountered in the coextrusion process, in maintaining appropriate flow velocities of the individual molten polymer streams and in setting the profile of the various layers at coextrusion to obtain a uniform thickness profile across the width of the extruded film. The individual films in the laminate must adhere during stretching yet be easily delaminated after orientation. Non-uniform adhesion between the layers during delamination can result in film breaks or wrinkles in the wound roll.
A typical example for improving the runnability of thin films by a process of the type described above is seen in European Patent Application 92107411.8. The method of the instant application for making and winding a self supporting thin film (an ultrathin film thickness of 2.5 microns or less) without the need for laminating reinforcement represents a distinct improvement over this process.
Film gauge variations also present a serious problem in winding up the film to obtain acceptable roll formation. For example, the thicker sections in a continuous length of film provide hard surfaces on the wound rolls while the thinner sections are soft. Such rolls have a tendency to wrinkle or telescope during handling. Furthermore, it is difficult to maintain an even tension upon such film when unwinding the roll, thus making slitting difficult. Moreover, film having relatively high gauge variations across its width is also difficult or impossible to handle in various types of converting equipment.
It is, therefore, important that these gauge variations be reduced, and that runnability be improved so as not to prevent or hinder the ready formation of smooth finished rolls for subsequent film processing such as vacuum metallization, coating and lamination.
Accordingly, a method has long been sought for making a biaxially oriented ultrathin film thickness of 2.5 microns or less with acceptably uniform gauge, in tenter frame, without confronting the problems of runnability and gauge uniformity as seen in the prior art.
The present invention provides such a method by surprisingly finding that it is possible to make extremely thin films of this type by using radiant heaters to maintain the temperature of the film at acceptable levels in the stretching zone. More specifically, it has been found that these heaters can be used to effectively maintain the proper temperature differentials between the thicker and thinner regions of thin film in thermal equilibrium with the surrounding air during stretching. If forced air heat is used, these levels cannot be readily maintained, particularly in stretching film having a cast thickness of less than 25 microns, as in this invention. By using these radiant heaters to heat the film during stretching, biaxially oriented ultrathin films having thickness of 2.5 microns or less can be readily made, using simultaneous stretching techniques with minimal runnability problems. Ultrathin films, having thickness as low as 0.2 microns, may be produced using this method. Such films, in acceptable commercial length of over 5000 meters, can be wound into a roll.